Various devices are known for forming a seal between a rotatable shaft, or a sleeve mounted on a rotatable shaft, and a housing or other structure surrounding the shaft. One type of seal, sometimes referred to as a contact circumferential shaft seal, is very effective in controlling leakage. These seals include one or more seal rings with circumferential inner faces that contact the rotating sleeve and slide against the sleeve while it rotates. Such seals may be formed from compacted and sintered carbon graphite to provide heat and wear resistance, and they are often formed as a plurality of interconnectable ring segments to facilitate installation about the sleeve. The seal rings are held in place by a suitable retaining device and may include a biasing device, such as a circumferential or garter spring, for holding the seal segments together.
While carbon seal rings are durable and capable of withstanding high levels of heat and friction, sliding contact with a rotating sleeve eventually causes the rings to wear out. The rate at which the carbon rings wear is based in part on the relative speed of the sleeve and shaft, and in some high-efficiency jet engines, this speed, expressed as a linear velocity, can exceed 600 feet/second or about 400 miles/hour, for extended periods of time. The heat generated by contact at such speeds causes the seal rings to wear and require frequent maintenance and/or replacement. The desire for longer operating life and higher thermal efficiency has therefore moved the seal industry to look for alternatives to circumferential contact seals.
One alternative to circumferential contact seals is a circumferential gas film seal. Much like the circumferential contact seal, this seal includes one or more carbon seal rings that exert a very light contact force against the rotating sleeve when it is rotating or not rotating. The light contact force is achieved by routing high pressure gas to opposing faces through clearance spaces and milled cutouts. In the case of a contacting circumferential seal, the outer diameter of the ring is exposed across its entire width while the inside diameter is exposed across its entire width except for the width of a small sealing dam. This creates an imbalance in force that lightly seats the seal against the rotating sleeve. Producing a force balanced contact in this manner is referred to hydrostatic sealing, and a hydrostatic seal can be maintained both when the shaft is rotating and when the shaft is stationary. Alternately or in addition, hydrodynamic sealing can be produced by forming recesses or cutouts on the side of the seal ring that faces the sleeve. As the sleeve rotates, air entrained by the rotating sleeve is compressed in these cutouts, and as it escapes over the non-recessed “pads” between the recesses, it produces an additional pressure and flow of air for maintaining a separation between the seal ring and the sleeve. Circumferential gas film seals generate less friction and less heat than circumferential contact seals, and thus generally last longer, require less maintenance and experience less oil cooling efficiency loss than contact seals.
One shortcoming of known circumferential gas film seals is that they are directional. In other words, the configuration of the recesses that produce the gas film when the sleeve rotates in a desired direction relative to the sleeve are not effective when the sleeve rotates in the opposite direction. Rotation in the “wrong” direction can occur either when the seal ring is installed backwards on the sleeve or when the sleeve rotates in the wrong direction inside a properly installed seal ring. Rotation in the wrong direction can occur, for example, when a strong wind blows through a jet engine and rotates the shaft backwards, an event referred to as “windmilling.”
When the sleeve rotates in the wrong direction relative to the seal, rather than lift the seal off the sleeve, the cutouts in the circumferential face may actually “suck” the seal toward the rotating sleeve and increase friction and the speed at which the seal ring wears. It would therefore be desirable to provide a bidirectional circumferential gas film seal that performs in a satisfactory manner independently of its mounting direction and/or when mounted on a rotatable sleeve that is intended to or at least capable of rotating in two directions.